[Haskell-cafe] Problem with benchmarking FFI calls with Criterion

[Janek S.]

Dnia wtorek, 27 listopada 2012, Jake McArthur napisał:
> I once had a problem like this. It turned out that my laptop was stepping
> the cpu clock rate down whenever it got warm. Disabling that feature in my
> BIOS fixed it. Your problem might be similar.
I just check - I disabled frequency scaling and results are the same. Actually I doubt that 39us 
of benchmarking would cause CPU overheating with such repeatibility. Besides, this wouldn't 
explain why the first benchmark actually got faster.

Janek

>
> On Nov 27, 2012 7:23 AM, "Janek S." <fremenzone at poczta.onet.pl> wrote:
> > I tested the same code on my second machine - Debian Squeeze (kernel
> > 2.6.32) with GHC 7.4.1 - and
> > the results are extremely surprising. At first I was unable to reproduce
> > the problem and got
> > consistent runtimes of about 107us:
> >
> > benchmarking FFI/C binding
> > mean: 107.3837 us, lb 107.2013 us, ub 107.5862 us, ci 0.950
> > std dev: 983.6046 ns, lb 822.6750 ns, ub 1.292724 us, ci 0.950
> >
> > benchmarking FFI/C binding
> > mean: 108.1152 us, lb 107.9457 us, ub 108.3052 us, ci 0.950
> > std dev: 916.2469 ns, lb 793.1004 ns, ub 1.122127 us, ci 0.950
> >
> > I started experimenting with the vector size and after bumping its size
> > to 32K elements I started
> > getting this:
> >
> > benchmarking FFI/C binding
> > mean: 38.50100 us, lb 36.71525 us, ub 46.87665 us, ci 0.950
> > std dev: 16.93131 us, lb 1.033678 us, ub 40.23900 us, ci 0.950
> > found 6 outliers among 100 samples (6.0%)
> >   3 (3.0%) low mild
> >   3 (3.0%) high severe
> > variance introduced by outliers: 98.921%
> > variance is severely inflated by outliers
> >
> > benchmarking FFI/C binding
> > mean: 209.9733 us, lb 209.5316 us, ub 210.4680 us, ci 0.950
> > std dev: 2.401398 us, lb 2.052981 us, ub 2.889688 us, ci 0.950
> >
> > First result is always about 39us (2,5 faster, despite longer signal!)
> > while the remaining
> > benchmarks take almost two times longer.
> >
> > Janek
> >
> > Dnia niedziela, 25 listopada 2012, Janek S. napisał:
> > > Well, it seems that this only happens on my machine. I will try to test
> > > this code on different computer and see if I can reproduce it.
> > >
> > > I don't think using existing vector is a good idea - it would make the
> >
> > code
> >
> > > impure.
> > >
> > > Janek
> > >
> > > Dnia sobota, 24 listopada 2012, Branimir Maksimovic napisał:
> > > > I don't see such behavior neither.ubuntu 12.10, ghc 7.4.2.
> > > > Perhaps this has to do with how malloc allocates /cachebehavior. If
> > > > you try not to allocate array rather use existing one perhaps there
> > > > would
> >
> > be
> >
> > > > no inconsistency?It looks to me that's about CPU cache performance.
> > > > Branimir
> > > >
> > > > > I'm using GHC 7.4.2 on x86_64 openSUSE Linux, kernel 2.6.37.6.
> > > > >
> > > > > Janek
> > > > >
> > > > > Dnia piątek, 23 listopada 2012, Edward Z. Yang napisał:
> > > > > > Running the sample code on GHC 7.4.2, I don't see the "one
> > > > > > fast, rest slow" behavior.  What version of GHC are you running?
> > > > > >
> > > > > > Edward
> > > > > >
> > > > > > Excerpts from Janek S.'s message of Fri Nov 23 13:42:03 -0500 2012:
> > > > > > > > What happens if you do the benchmark without unsafePerformIO
> > > > > > > > involved?
> > > > > > >
> > > > > > > I removed unsafePerformIO, changed copy to have type Vector
> >
> > Double
> >
> > > > > > > -> IO (Vector Double) and modified benchmarks like this:
> > > > > > >
> > > > > > > bench "C binding" $ whnfIO (copy signal)
> > > > > > >
> > > > > > > I see no difference - one benchmark runs fast, remaining ones
> > > > > > > run slow.
> > > > > > >
> > > > > > > Janek
> > > > > > >
> > > > > > > > Excerpts from Janek S.'s message of Fri Nov 23 10:44:15 -0500
> >
> > 2012:
> > > > > > > > > I am using Criterion library to benchmark C code called via
> >
> > FFI
> >
> > > > > > > > > bindings and I've ran into a problem that looks like a bug.
> > > > > > > > >
> > > > > > > > > The first benchmark that uses FFI runs correctly, but
> > > > > > > > > subsequent benchmarks run much longer. I created demo code
> > > > > > > > > (about 50 lines, available at github:
> > > > > > > > > https://gist.github.com/4135698 ) in which C function
> >
> > copies a
> >
> > > > > > > > > vector of doubles. I benchmark that function a couple of
> >
> > times.
> >
> > > > > > > > > First run results in avarage time of about 17us, subsequent
> > > > > > > > > runs take about 45us. In my real code additional time was
> >
> > about
> >
> > > > > > > > > 15us and it seemed to be a constant factor, not relative to
> > > > > > > > > "correct" run time. The surprising thing is that if my C
> > > > > > > > > function only allocates memory and does no copying:
> > > > > > > > >
> > > > > > > > > double* c_copy( double* inArr, int arrLen ) {
> > > > > > > > >   double* outArr = malloc( arrLen * sizeof( double ) );
> > > > > > > > >
> > > > > > > > >   return outArr;
> > > > > > > > > }
> > > > > > > > >
> > > > > > > > > then all is well - all runs take similar amount of time. I
> >
> > also
> >
> > > > > > > > > noticed that sometimes in my demo code all runs take about
> > > > > > > > > 45us, but this does not seem to happen in my real code -
> >
> > first
> >
> > > > > > > > > run is always shorter.
> > > > > > > > >
> > > > > > > > > Does anyone have an idea what is going on?
> > > > > > > > >
> > > > > > > > > Janek
> > > > >
> > > > > _______________________________________________
> > > > > Haskell-Cafe mailing list
> > > > > Haskell-Cafe at haskell.org
> > > > > http://www.haskell.org/mailman/listinfo/haskell-cafe
> > >
> > > _______________________________________________
> > > Haskell-Cafe mailing list
> > > Haskell-Cafe at haskell.org
> > > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe